The present invention concerns the display of radar images and, more particularly, a case where one and the same geographical zone is covered by several radars and where it is desired to obtain a single resultant image, formed by a mosaic of the images given by the different radars.
The need to make a mosaic arises notably when the position of a radar is such that it cannot cover the entire zone it is entrusted with monitoring, owing to obstacles such as mountains or tall buildings creating a shadow in the zone under surveillance, namely a sector without echo. This, for example, is often the case with airport surveillance radars.
To cover the entire zone, one or more additional radars are then needed, and are arranged so as to reach the shaded sectors of the first radar.
To enable an operator to have a comprehensive image of the zone on a single screen, it is then necessary to make a mosaic of images given by the different radars.
There are known ways to make this mosaic by taking, for each of the pixels of the resultant image, a video signal equal to a function (mean, maximum etc.) of each of the pixels given by the different radars. Since, in general, the images given by the different radars overlap one another, two types of zones are then distinguished on the single screen:
those reached by a single radar; PA0 those reached by several (at least two) radars.
In the second type of zone, the echos of the detected objects (aircraft for example) processed by each of the radars should, of course, be superimposed on the screen and, to a given aircraft, there should therefore. This requiremement may not necessarily be achieved due to the asynchronism of the rotation of the radars. There may be movement by the aircraft between the instant when it is detected by the first radar and the instant when it is detected by the second one.
Furthermore, the radar video signal always contains a certain degree of noise and, with this type of system, the noises get added together in the zones common to several radars.